The portable devices, such as contactless smart labels (tags), are widely used today for identifying any objects by a remote reader.
Generally, the label also known as a radiofrequency identification device (RFID) is used to recognize or identify, at a fairly large distance and within a very short time, an object, an animal or a person carrying the label. Such an electronic label essentially consists of an integrated circuit containing an electronic identification device in which the data that is part of the considered application is stored in memory and an antenna connected to the integrated circuit. The information contained in the chip is read by a reader, which transmits electromagnetic signals at a given frequency to the label. The label being a passive device, the reader provides it with power enabling the RFID device to transmit in return the contents of the memory such as the identification number. A dialog is established according to a predefined communication protocol and a certain amount of data is thus exchanged between the label and the reader.
The communication between the label and the reader cannot be established without a minimum of energy. The quality and quantity of energy transferred depend on certain criteria such as the operating frequencies, the distance between the antenna of the reader and the antenna of the label, etc. The label, considered as a resonant circuit, is tuned at a given rated frequency so that there is optimum communication between it and the antenna of the reader when the label is located in the field emitted by the reader. Currently, the electromagnetic signals transmitted by the reader are in UHF mode within 860-960 MHz and 2.45 GHz frequency ranges. Transmissions in UHF mode allow exchanges at a large distance, in the order of 50 cm to 1 m or several meters, as the amount of energy required for the exchange of information between the label and the reader is very small.
FIG. 1 represents the circuit diagram of a traditional RFID device including an antenna 10 connected to a chip 12. The electromagnetic signals received are rectified by the diodes 15 and 16. The capacitance 14 is used as AC link to UHF signals. As for the polarized capacitor 18, it plays the role of a reservoir capacitance to provide the supply voltage to the chip. All the other components of the chip are equivalent to an impedance 20.
When the RFID device transmits its digital identification data to the reader, it does it by means of an electronic switch 22, at a rate linked to the output. The closure of this “switch” acts as a mismatch of the Chip—Antenna assembly. In this way a different quantity of energy is absorbed and a different quantity of energy is reflected towards the reader. A retromodulation subcarrier frequency may also be applied by modulating this subcarrier by the open or closed position of the switch 22 defined by the digital identification data to be transmitted to the reader.
The RFID device shown in FIG. 1 has a major drawback. When the switch 22 is open, a voltage is provided to, and thus charges, the reservoir capacitance 18. But when the switch 22 is closed, even if there actually exists a non-zero impedance of the switch, it is negligible and the power received by the antenna 10 is almost entirely reflected. Only a minute quantity of this energy is supplied to the capacitance 18. The latter providing the supply voltage for data transmission therefore discharges. We can thus consider that diagram giving the voltage as a function of time at the terminals of the capacitance 18 is that shown in FIG. 2. On opening the switch 22, the capacitance discharges and changes from a minimum level Vmin at time T1 to a maximum level Vmax at time T2. Then, on closing the switch, the capacitance once again discharges from Vmax at time T2 to Vmin at time T3, and so on.
If the RFID device is quite far away from the reader, the power received is low. With an equal capacitance, the voltage at the terminals of the capacitor 18 then drops below a threshold voltage Vthreshold required for the circuits of the chip to operate and for the identification data to be transmitted.
In order to offset this drawback, the solution used at present is to increase the capacitance if distant objects have to be identified. However, we then encounter the effective area of the chip. Assuming that the capacitance is in the order of 200 pF and accepting a large capacitance per unit area in the order of 6 fF/μm2, a capacitance surface of 33330 μm2 is required, i.e. already more than 3% of the surface area of a chip which would occupy approximately 1 mm2! Increasing the capacitance is therefore counterproductive regarding miniaturization which is an essential factor in this field. This drawback becomes an even greater handicap when the size of the chip decreases.